Table 1.

Body Mass Index (BMI).

BMI is a common measure of weight adjusted for height:

$\frac{\text{Weight(kg)}}{{\left\{\text{Height(m)}\right\}}^2}=\frac{\text{Weight(lb)}\times \text{703}}{{\left\{\text{Height(in)}\right\}}^2}$

While BMI does not measure body fat directly, it is an inexpensive and easy method of screening for potential obesity-related health risks and surveillance of population-level childhood obesity. BMI has been correlated with direct measures of body fat, including skinfold thickness and dual x-ray absorptiometry. (Kelly et al., 2013, Little and Rubin, 2002) Thus, using BMI to define obesity has several advantages over other more invasive, if precise, metrics.

BMI categories for adults are fixed, but for children aged 2 through 19 years, BMI is expressed using percentiles that take into account factors such as age-in-months, sex, growth, and level of secondary sexual maturation, that affect the relationship between BMI and body fat among children. In the US, percentiles specific to age and sex are calculated from CDC growth charts (Plowman and Meredith, 2013) which compare a given BMI to the BMI values of other US children of the same sex and age that were surveyed from national data collected from 1963 to 65 to 1988–94. As defined by the CDC, a child’s weight status can be underweight (BMI-for-age < 5th percentile), healthy weight (BMI-for-age ≥ 5th and < 85th percentile), overweight (BMI-for-age ≥ 85th and < 95th percentile); obese (BMI-for-age ≥ 95th percentile); and severely (or extremely) obese (BMI-for-age ≥ 120% of the 95th percentile). (Langkamp et al., 2010, Bezold et al., 2014) The definition for severe (or extreme) obesity has been recommended by the American Heart Association (AHA) as a flexible means by which to evaluate heavier youth (CDC. A SAS Program for the, 2000) and is approximately equal to the empirical 99th percentile in the CDC growth charts. (California Department of Education. Physical Fitness Test Results (CA Dept of Education). Accessed July 8, 2020)